The disclosure of Japanese Patent Application No. HEI 10-367236 filed on Dec. 24, 1998, including the specification, drawings, and abstract thereof, is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an air-fuel ratio detecting apparatus and method employing a limit current type air-fuel ratio sensor and, more particularly, to fault diagnosis of an air-fuel ratio detecting apparatus.
2. Description of the Related Art
In order to achieve both a reduced specific fuel consumption and a reduced amount of harmful gas emissions in a vehicular internal combustion engine, it is necessary to control the air-fuel ratio to be burned in the engine over a wide air-fuel ratio range. To enable such air-fuel ratio control, an air-fuel ratio sensor (generally termed full-range air-fuel ratio sensor, linear air-fuel ratio sensor, and the like) utilizing the occurrence of a limit current in accordance with an oxygen concentration or an unburned gas concentration in exhaust gas in response to application of a current to a sensor body provided by forming an atmospheric-side electrode, an exhaust-side electrode and an exhaust-side diffusion resistor in an oxygen ion-conductive element of, for example, a zirconia solid electrolyte or the like, is known. Air-fuel ratio feedback control based on outputs of such air-fuel ratio sensors is performed according to the conventional art.
Air-fuel ratio feedback control based on the output of a full-range air-fuel ratio sensor essentially requires that an active state of the oxygen ion-conductive element be maintained. Therefore, the conventional art performs a control in which the temperature of the oxygen ion-conductive element is kept constant by heating the element through the use of a heater. Although the element temperature is a necessary factor in this control, the need for a temperature sensor is eliminated by estimating an element temperature from a detected element resistance on the basis of a correlation between the element resistance and the element temperature.
It is a precondition for the use of the above-described air-fuel ratio sensor to perform fault diagnosis on the air-fuel ratio sensor and, if there is any abnormality, take an immediate measure. Japanese Patent Application Laid-Open No. HEI 8-271475, as for example, discloses a technology that detects an element resistance of an oxygen ion-conductive element and, if the element resistance is outside a predetermined range, determines that the oxygen ion-conductive element has an abnormality.
However, since the air-fuel ratio sensor is formed by sensor body, a sensor body drive circuit for driving the sensor body and detecting an output current thereof, and a heater and heater drive circuit for heating the sensor body to activate the oxygen ion-conductive element, it is necessary to consider a wiring break or the like in the sensor body drive circuit, a wiring break or the like in the heater or the heater drive circuit, deterioration of the heater, and the like.
Accordingly, it is an object of the invention to provide an air-fuel ratio detecting apparatus that is formed by an air-fuel ratio sensor and its peripheral circuits so as to be capable of performing fault diagnosis in a simple, easy, and accurate manner.
To achieve the aforementioned and other objects of the invention, an air-fuel ratio detecting apparatus in accordance with one aspect of the invention includes an air-fuel ratio sensor having a sensor body that generates a limit current in accordance with at least one of an oxygen concentration and an unburned gas concentration in a detection-object gas upon application of a voltage to the sensor body, a sensor body drive circuit that drives the sensor body, an element impedance detection device for detecting an element impedance of an oxygen ion-conductive element provided in the sensor body by applying an AC voltage to the sensor body, and a diagnostic device for, based on the element impedance detected by the element impedance detection device, determining whether there is a fault and diagnosing a fault.
In the air-fuel ratio detecting apparatus of the invention, if the element impedance is greater than a first reference value and is considered to be infinite, the diagnostic device may determine that there is a break in at least one of the sensor body and the sensor body drive circuit.
Furthermore, the air-fuel ratio detecting apparatus may further have a construction as follows. The air-fuel ratio sensor has a heater that heats the sensor body to activate the oxygen ion-conductive element. The air-fuel ratio detecting apparatus further includes a heater drive circuit that drives the heater. If the element impedance is smaller than the first reference value but greater than a second reference value, the diagnostic device determines that there is an abnormality in at least one of the heater and the heater drive circuit.
Still further, if the element impedance is smaller than a third reference value that is between the first reference value and the second reference value in terms of magnitude and the element impedance is greater than the second value, the diagnostic device may determine that the heater has deteriorated.
Further, if the element impedance is greater than a third reference value that is between the first reference value and the second reference value in terms of magnitude and the element impedance is smaller than the first value, the diagnostic device may determine that there is a break in at least one of the heater and the heater drive circuit.
Further, the diagnostic device may perform diagnosis when the oxygen ion-conductive element is in an active state.
Further, the diagnostic device may perform diagnosis when an internal combustion engine in which the air-fuel ratio sensor is installed is in an idle state.
Further, the element impedance detection device may detect the element impedance by applying a voltage obtained by superimposing an element impedance-detecting AC component on an air-fuel ratio-detecting DC component, to the sensor body, and measuring an AC component of an output current of the air-fuel ratio sensor.